Running thru the last smashwords survey of ebook sales (a yearly summary of ebooks which provides useful information) always provides interest insights. Here's the list of this years most popular genres:
1: Romance (47% of the total market)
4: Young adult or teen
5: Science Fiction
6: Gay and lesbian
7: Thriller and suspense
11: Literature (not sure how this is defined)
12: Women's fiction (stories about women or about women's issues?)
13: African American fiction
15: Children's books
17: Humor & comedy
That shows that SOL is on the right track by frequently combining romance and erotica (thought mostly from a male perspective), though we frequently miss the YA market, and almost completely ignore the #6 category entirely.
It also hints that a decent way to boost flagging thriller sales is by specializing (combining #6 and #7, and maybe even #8). Combining #14 and #13 or #15 is largely as wasted effort, on the other hand.
The actual sales by these categories are:
Historical Fiction/Literary Fiction/Thriller/suspense (each 2%)
Aside from being the top sellers, romance readers are the typically book-a-day readers, romance authors are typically the first to adapt new marketing practices and there are more professional romance associations and conferences.
Strangely (or maybe not) the top romance & erotica subcategories are:
#3 Paranormal and #4 Couples Erotica
No surprise is #8 Lesbian erotica and #10 Sci-fi erotica. MILF, Time travel historical and horror are the lowest romance categories. However, the nebulous "Contemporary" romance is the far-and-away best selling romance, across the board.
A big surprise for me were "box sets", or multiple books combined into a single book (say an entire series sold as a single volume). The average price of these was $7.43 and the word count was 316,644. However, it's offers the fewest best-sellers (of 3 of the top 100 bestsellers).
$2.99 is the most common price point, followed by $4.99, $9.99 and $3.99 (the suggestion being that if you have strong sales at $2.99, you should migrate to $3.99 or $4.99). $3.99 seems to get the most downloads, though, followed by $4.99. (The higher prices reflect mostly non-fiction works.)
Pre-orders seem to work best for Romance, YA, Gay/Lesbian and historical fiction, but performs badly for erotica.
For book series: 7 of the 10 bestsellers had free series starters, and 7 of 10 were romance (2 YA). The average series contains 7.48 titles, while the medium is 6).
Series also seems to boost overall sales by 383% over single books sales.